As is well known in the art, the effective load movement of such a crane depends not only on the weight of a load suspended from its hoisting cable but also on the length of its boom and its angle of inclination or elevation. The point at which this moment begins to impair the stability of the crane, when the same is designed as a mobile vehicle, is also determined in each instance by the dimensions of its base and, at least in the absence of lateral outrigger-type supports, by the azimuthal angle included between the boom and the vehicular axis. Reference in this connection may be made, for example, to U.S. Pat. Nos. 3,638,211 and 3,740,534 as well as others listed therein as cited references.
From the patents referred to above it is known to provide such a crane with a computer including a memory digitally storing information peculiar thereto, the memory being addressable by a boom-angle sensor and a boom-length sensor to read out a numerical value representing the maximum load that can be safely supported under the existing operating conditions. This value is compared with the magnitude of the load actually measured and an alarm signal is given if the measured magnitude approaches the stored value.
Such a memory must have a large storage capacity in view of the many combinations of boom length, boom angle and possibly other parameters that have to be taken into account.
It is also known, e.g. from British Pat. No. 1,107,116, to use an analog computer for the purpose of determining the maximum permissible load moment on the basis of signals measuring such parameters as the boom angle and the tension of a hoisting cable supporting the load. As more particularly described in that British patent, the computer may operate along a straight line or a curve--e.g. an arc of a circle or a parabola--which approximates a curve representing the permissible maximum load as a function of angle of elevation. A path corresponding to that straight line or curve can be traced by a lever or a slide coacting with suitable markings.
A straight line or even a second-order curve such as a parabola, however, is only a rather rought approximation of the actual load characteristic of a crane of the kind here envisaged. In order to satisfy national and international regulations concerning crane safety, a control system of this simplified type would have to be so designed that the deviations lie on the "safe" side of that characteristic; this entails an underutilization of the load-carrying capacity in many instances.